I have a Rem. 700 PSS (Police Sharp Shooter)in .223. Heavy HS Enterprises stock. Scope is a Springfield Armory 4 - 14x 40mm, 80 mm scope hood. 2 lb trigger. Shoots .4 MOA at 100 yds. (Probably tighter, I can't shoot tighter.)
It's a trade in from the Oregon State Police who have opted for the 308.
The scope is calibrated to 700 yds. That's with 55 gr. bullet @ 3000 fps. The trick to making this an effective "sniper" caliber is to shoot 69 gr. match grade (Sierra) BTHP bullets. I run 26 gr. Varget for about 3,050 fps.
Also, I fire form the brass and neck size only. That way the brass fits the chamber of MY rifle.
We turn the necks to get them concentric. Sinclair has a tool for this.
OAL is figured with a Sinclair OAL gauge and bullet comparitor.
Uniform the primer pockets, deburr the flash hole. Trim the neck, deburr and polish with 0000 steel wool in a polishing cup.
I toss the charge from a Uniflow measure and then use a trickler to get it on the mark with a beam scale. Double check this measure with an RCBS (Pact) digital electronic scale.
We don't tumble the brass. That nicks the mouths and work hardens the necks. Brass gets hand cleaned. Necks get annealed after five firings.
Brass is sorted into lots by weight, brand, lot number, and number of times fired.
Bullets are sorted by length and weight. (These are match grade to begin with.)
I spend five or six hrs loading 50 rds. And I don't moly coat . . . that's more time.
The bore gets swabbed before shooting, then after every five-shot group. No rapid fire to heat up the bbl. Lots of tools to carry to the range . . .
"Sniper" or bench shooting is mostly about loading accurate ammo, and shooting a well prepared gun. Semi-autos don't afford the precision of a bolt action "sniper/bench/varmint" rifle.
Varmint Al's has a reloading page that describes the reload process:
http://www.cctrap.com/~varmint/arelo.htm
accuratereloading.com is another good site with excellent links.
The AR with a floating bbl and a flat top mounted scope is a good tactical/accuracy gun, but you can't fire form and fine tune the brass.
The JP triggers are nice. JP also makes a pin for tightening up the fit between upper and lower.
You'd have a straight shooter, probably sub MOA. But it's not going to serve as a "sniper" gun in the true sense of the word.
That said, I use the Rem. for varmints like sage rats, and the AR (Bushmaster Commando, floated bbl. Redfield 6x scope, Jewell trigger) for coyotes. Mostly because the dogs tend to run, and I can get off a second, third . . . sixth shot if necessary. It's accurate off the bench, but not like the Remington.